The document published under the number FR 2960303 describes one such neutron detection apparatus. Said detection apparatus comprises an enclosure forming a cathode filled with gas and comprising a hollow body. Said hollow body comprises two main walls joined together by lateral walls and having two substantially parallel internal surfaces and covered by a solid layer containing boron or a compound of boron. A device forming an anode extends inside the enclosure. Said apparatus also comprises at least one intercalary wall forming a cathode fixed on the lateral walls and extending inside the enclosure substantially parallel to the internal surfaces of the hollow body. The or each intercalary wall receives a solid layer containing boron or a compound of boron.
The method for making the solid layer of boron proposed by the document is the depositing or projecting of a boronated suspension, also called boron nanoparticle-based ink, onto the supports whose surfaces must be covered by a solid layer of boron. The support is heated beforehand so that the ink dries immediately without having time to run laterally. Furthermore, a surfactant is mixed with the ink to ensure a homogeneous deposit and to improve the adhesion of nanoparticles on the surface of the support. However, this method does not guarantee homogeneity of the thickness of the solid layer of boron nor satisfactory fixing of said layer. Indeed, if for example the ink does not dry rather quickly, the thickness of the layer on the bottommost part of the support in relation to the ground during drying will be greater than on the uppermost part. A perfectly horizontal position of the support might be a solution, but it will nevertheless be necessary to add surfactant in proportions of 30% to 40% to ensure that the nanoparticles adhere to the support. The addition of this surfactant reduces the efficacy of the layer of boron and thus of the detection apparatus.
Other methods of depositing a solid layer of boron exist, such as cathode sputtering, vapor phase deposition, or deposition by thermal dissociation of boranes. But, the cathode sputtering method requires long treatment times and the layer obtained is not stable and disintegrates easily. The vapor-phase deposition method requires very high temperatures in the vicinity of the vaporization temperature of boron, about 3927° C. The deposition method using thermal dissociation of boranes requires the use of gases that are both explosive and toxic. Furthermore, this deposition method and the vapor-phase deposition method are costly because they require the use of 3 g to 5 g of boron 10 (10B) per gram deposited. However, 10B is very expensive and a neutron detection apparatus needs about 50 g of 10B deposited.
The deposition of a solid layer of boron is thus a tricky point in the manufacture of a neutron detection apparatus. In order for the apparatus to have satisfactory efficacy, the layers of boron are essentially composed of 10B of controlled thickness of between 0.03 mg/cm2 and 0.5 mg/cm2. The density of natural boron is 2.34 and that of 10B is slightly lower and equal to 2.16. A value of 0.1 mg/cm2 thus corresponds to a thickness of 460 nm and 0.4 mg/cm2 to 1850 nm. The thickness must be controlled within about 20%. Furthermore, the proportion of 10B in the layer must be as close as possible to 100%, which not only imposes the use of 10B enriched boron but also strongly limits the use of additives such as surfactants, dispersants or adhesives.